Multiple Input Multiple Output (MIMO) systems are becoming popular in wireless communications to leverage aspects of intersymbol interference to potentially increase the bandwidth efficiency of existing spectra. In wireless communication, radio waves do not propagate in a straight line between the transmitter and receiver, but rather bounce and scatter randomly off objects in the environment. This scattering, known as multipath, results in multiple copies of the transmitted signal arriving at the receiver via different scatter paths. MIMO leverages multipath to enhance transmission accuracy and allow multiple signals to be broadcast at the same frequency. This is done by treating the multiple scatter paths as separate parallel sub channels, each capable of bearing distinct data.
MIMO operates by splitting a discrete outbound signal into multiple substreams using an array of transmitter antennas to simultaneously launch the parallel substreams. The outbound signal may be de-multiplexed into a number of parallel data streams or multiple copies of the outbound signal may be transmitted by the antennas. All the substreams are transmitted in the same frequency band, so spectrum is efficiently utilized. Another array of antennas in the receiving device, is used to pick up the multiple transmitted substreams and their scattered signals. Each receive antenna picks up all of the incident transmitted substreams superimposed as observed components of the received signal vector, not separately. However, the multiple substreams are all scattered slightly differently, since they originate from different transmit antennas that are located at different points in space. These scattering differences allow the substreams to be identified and recovered from the observed components of the received signal vector.
While MIMO facilitates efficient utilization of spectrum, it presents additional challenges in detecting the signal substreams intended for a given receiving device. For example, signals intended for other receiving devices may be broadcast simultaneously or nearly simultaneously in the same frequency band by the same transmitter array. In addition, neighboring transmitter arrays also transmit in the same frequency band.